Modern accelerometers, pressure transducers, and similar transducers are often fabricated as Micro Electro-Mechanical System (MEMS) sensor and actuator devices. It is conventional in the electronic industry to encapsulate such MEMS device mechanisms in chip carrier packages. Chip carrier packages usually seal a semiconductor or MEMS sensor device chip within insulative plastics or resins. This protects the chip from environmental hazards as well as providing a means for electrically and mechanically attaching the chip to an intended device. A primary focus in the design of such a chip carrier package is to provide the chip with adequate protection from the external environment during handling and operation.
Ceramic and metal chip carrier packages are considered hermetic. However, chip carrier package include designs for total encasement chip carrier packages, commonly referred to as “TE packages.” The TE package generally includes a lead frame having a chip mounting pad, an integrated circuit chip which is attached to the chip mounting pad, a plurality of fragile wires which connect the chip to the lead frame, and a thermoset plastic which totally encases the lead frame, the chip and the plurality of fragile wires. It may also include a heat sink (HS) or similar heat dissipation means.
Thermotropic liquid crystal polymers, such as polyesters and combinations of different polyesters, commonly referred to as LCP, are the thermoplastic materials often used in the electronics industry for TE packaging. When used as a TE package, LCP is usually used as the body of the package, the “bathtub,” to which a glass, ceramic or metal lid is sealed by heating the LCP until it melts and flows to produce a bond between the LCP and the lid.
LCP is also the thermoplastic material often used in electronics packaging for flex tape substrates where a metal electrical conductor (e.g., copper, aluminum, silver, and the like) is co-extruded with LCP film or sheet to form an LCP film suitable for multilayer laminates, e.g., circuit boards. The LCP is heated until it melts and flows to produce a bond between the LCP and metal laminated layers. Signals come out of the TE package on metal electrical conductors between laminated layers of LCP. As a result, the majority of the package is LCP.
In contrast to glass, ceramic and metal, LCP materials are generally considered semi-permeable. Use of LCP causes TE packages to be only semi-hermetic, even when sealed with a glass, ceramic or metal lid. LCP material is not nearly as permeable as most plastics, and so can be a suitable chip carrier package for many electronic and MEMS devices. Some MEMS sensors previously thought to require fully hermetic packages are now produced with semi-hermetic packages, especially in commercial and automotive applications. However, some MEMS sensor and actuator devices are still limited to hermetically sealed chip carrier packages.